To those of you badgering the game developer to allow you to kill guards I have a simple and understandable explanation. You are an assassin sent from the future. What sort of repercussions would occur from killing mindless guards who have families? Such destruction of time and subsequent paradoxes have to be avoided.
So... You're a assassin from the future who can go invisible and see the line of sight of any guards with some unknown technology, yet you are only outfitted with a knife and your suit isn't bullet proof?!